1. Field of the Invention
The present invention relates to a solid-state imaging device and a method for manufacturing the same.
2. Description of the Related Art
For a solid-state imaging device such as a CMOS (complementary metal oxide semiconductor) sensor (CMOS solid-state imaging device), the potential design for defining the charge accumulation region of the photodiode in each pixel is becoming difficult as the miniaturization of the pixels progresses.
The reasons why the potential design becomes difficult are as follows.
Of visible light beams, red light exhibits a low absorption coefficient in silicon. Therefore, photoelectric conversion thereof is performed also in a comparatively deep part of the photodiode.
Therefore, potential needs to be formed in a deep part of silicon in order to enhance the sensitivity to the red light.
Forming the deep potential requires ion implantation with high energy.
For the ion implantation with high energy, the resist used as the mask of the ion implantation needs to have large thickness.
However, because the resist formed as the mask of etching needs to have large thickness, the resist as the mask has a higher aspect ratio along with the miniaturization.
Therefore, it is becoming difficult to form the resist mask with high accuracy.
In addition, when the aspect ratio of the resist as the mask is higher, a lower part of the resist is thinner and thus the resist tends to fall down more easily, which causes the lowering of the manufacturing yield.
Furthermore, the profile of ion implantation with higher energy has wider lateral broadening.
Therefore, as the miniaturization of the pixels progresses, the isolation becomes more difficult and implanted impurity ions spread to adjacent pixels more frequently.
Because the ion implantation with high energy is difficult, red light, which reaches a deep part of silicon, exhibits a low rate of photoelectric conversion in the charge accumulation region, which causes sensitivity deterioration and color mixing.
To address this problem, there has been proposed a structure in which a reflective film is provided under the photodiode in each pixel so that light that has reached an area deeper than the photodiode may be reflected and returned to the photodiode (refer to e.g. Japanese Patent Laid-Open No. 2007-27604 (hereinafter, Patent Document 1)).
This structure eliminates the need to form the photodiode having large depth and thus also eliminates the need for the ion implantation with high energy.